A hollow tubular plastic core for an elastically shrinkable tubular cover must have sufficient strength to resist the shrinkage force of the cover member and retain its tubular shape, and it must be easily removed by breaking into a continuous strip with simple manual work. Known hollow plastic cores for elastically shrinkable tubular covers include one in which a continuous helical cut is made in the wall along the entire length of a tube, as disclosed in U.S. Pat. No. 3,515,798, and one in which a plastic strip is wound in a helix to make a tubular form with the edges of the strip abutted, and the abutted edges being ultrasonically spot welded.
It has been found to be impractical to make the tubular plastic core by making a continuous helical cut in a tube having a uniform wall thickness because of the problems of maintaining the cut at a given depth due to variations in the roundness and the wall thickness of the core and the movement of a cutter in cutting. A cut that is too deep does not provide sufficient strength to resist the shrinkage force of the tubular cover member or to prevent the tubular structure from deforming the core which might not permit a cable to be inserted. A cut that is too shallow does not permit the core to be manually broken into a strip with sufficient ease.
It is necessary in the process of forming a plastic strip into a tube by abutting its edges to be certain that the edges align properly, and the manufacturing process is, therefore, complex. Furthermore, due to its helical winding, the core can be broken into a strip only in one direction (an attempt to break it in the opposite direction will get the core tangled). The directional property of such a plastic core has to be taken into consideration in use, causing inconvenience in certain applications.